1. Field of the Invention
The present invention relates to a method for the remediation of soil containing contaminants such as organic halide compounds, boron containing-compounds, chromium-containing compounds, arsenic-containing compounds, nitric acid, salt thereof, nitrous acid and salt thereof; and an agent for remediation employable for the method.
2. Description of the Related Art
As contaminants (contaminated substances) contained in soil or groundwater, there have been known various substances, for examples, organic halides such as organic chlorides, compounds of heavy metal such as hexavalent chromium (Cr6+), arsenic-containing compounds, nitrogen in the form of nitric acid or nitrous acid and boron containing-compounds. Even if these contaminants are soluble in water, all of them do not flow out to reach the groundwater. They usually remain in the soil in the form of their various compounds, and slowly flow into the groundwater. Otherwise, when the contaminants adhere to the soil, most of them scarcely flow into the groundwater. Remediation of the contaminated soil and groundwater requires much more cost and time compared with those in method of preventing the contamination from its occurring. Therefore, the efficient method for remediation that enables reduction of cost and time is greatly desired.
For instance, volatile organic halides such as trichloroethylene that has been employed in a large amount for industrially washing various machines to remove stain-containing oils, have been increasingly regulated from the consideration of environment pollution. However, a large amount of organic halides already have been employed, and therefore soil and groundwater pollutions have been widely advanced. In more detail, the organic halides such as trichloroethylene are stable and scarcely decomposed by microorganisms, and hence the organic halides abandoned in the soil not only contaminate the soil per se, but also finally do the river or groundwater that is occasionally source for drinking water.
Examples of methods for remedying the soil contaminated with the volatile organic compounds such as organic halides include soil-gas sucking method, groundwater pumping-up method, and soil-digging method. The soil-gas sucking method is of coercively sucking aimed substances existed in an unsaturated zone (layer). This method comprises providing wells for suction in the soil (land formation) by boring, reducing the pressure within the wells by a suction pump, collecting the vaporized organic compounds into the wells, and leading the compound to the underground to cause them to be adsorbed by activated carbon for treatment. In the case that the contamination by the organic compounds extends to an aquifer, adopted is a method comprising providing a water pump within the suction wells and pumping up water together with soil-gas to treat them.
The groundwater pumping-up method is performed by providing pumping-up wells within the soil and pumping up contaminated groundwater to treat it. Further, the soil-digging method is done by digging the contaminated soil and subjecting the dug soil to a wind drying and heat treatment to remove and collect the organic compounds.
In the soil-gas sucking method and groundwater pumping-up method of these conventional methods, only water or air is sucked and removed, which results in formation of passages for water or air in the soil. Therefore, the methods have low efficiency of remediation and require a long term (e.g., several to ten years) for removal of contaminants. Moreover, the soil-digging method requires considerable cost when it is applied to a wide area.
When the soil or groundwater is contaminated by the organic halides, there are also in many times that they are done by various substances, e.g., boron containing-compounds, chromium-containing compounds, arsenic-containing compounds, nitric acid, salt thereof, nitrous acid and salt thereof (nitrogen derived from nitric or nitrous acid). It is desirable that the method for the remediation is further effective in the above substances other than the organic halides.
The object of the present invention is to provide the method for the remediation of soil containing contaminants (generally water-soluble substances) such as organic halide compounds in which the contaminants can be removed in high efficiency, short period and low cost; and the agent for remediation employable for the method.
In the conventional methods for the remediation of soil contaminated with the organic halides, the organic halides which has been brought together with rain water etc. into in the groundwater is removed in the form of groundwater containing the organic halides; or the organic halides in the soil is sucked to remove. However, the contaminants adhering to the soil cannot be easily removed. Otherwise, nonvolatile contaminants are not effectively removed by the suction method.
The inventors have been studied for obtaining an agent for remediation which is capable of removing efficiently contaminants adhering to soil and which does not cause secondary contamination. In the course of the study, they have noticed xe2x80x9cCarbonated Water Corrosion Effect against Mineralsxe2x80x9d, Y. Kitano, Kyodai Bosai-Ken Nenkan (Prevention of disasters yearbook of Kyoto University), Vol. 10A, 557 (1967), and further studied based on the effect. As a result, they have found that the carbonated water (carbonic acid aqueous solution) effectively liberates the contaminants in water from the soil (especially the limited concentration), to attain to the present invention.
The above-mentioned objects are attained by:
a method for the remediation of contaminated soil comprising causing a carbonic acid aqueous solution to penetrate into said soil to liberate and remove contaminants contained in the soil;
a method for the remediation of contaminated soil comprising providing wells for feeding carbonic acid and wells for pumping water with a distance between the wells in the soil by digging the soil, introducing a carbonic acid aqueous solution or carbon dioxide gas into the wells for feeding carbonic acid to liberate contaminants contained in the soil, and pumping up water from the wells for pumping water to remove the liberated contaminants; and
a method for the remediation of contaminated soil comprising providing a water cutoff wall for surrounding the soil, providing wells for feeding carbonic acid and wells for pumping with a distance between the wells within the surrounded area by digging the soil, introducing carbonic acid aqueous solution or carbon dioxide gas into the wells for feeding carbonic acid to elevate a level of groundwater in the wells and to wet the soil to the surface thereof, whereby contaminants contained in the soil are liberated, and pumping up water from the wells for pumping water to remove the liberated contaminants.
In the above-mentioned methods, the contaminants contained in the soil are preferably eluted in the carbonic acid aqueous solution. The contaminants contained in the soil preferably comprise at least one selected from the group consisting of an organic halide compound, a boron containing-compound, chromium-containing compound, an arsenic-containing compound, nitric acid, nitrate, nitrous acid and nitrite. These contaminants have a common property soluble in water. In more detail, the inventors have found that the organic halides are also soluble to some extent in water. Further, it is preferred that in the case of using the carbonic acid aqueous solution, the plural solutions (i.e., dilute and concentrated solutions) are alternately introduced (e.g., poured) or caused to penetrate into the soil with different concentrations from each other, and that in the case of using the carbon oxide gas, the gas is introduced into the soil at different feeding rates alternately. The introducing or penetrating of the carbonic acid aqueous solution preferably comprises a step of penetrating or introducing the solution and a step of then passing it through the soil. It is preferred that water is poured into the soil before introduction of the carbonic acid aqueous solution into the soil. Further, it is preferred that salt (NaCl) is added before pouring of the carbonic acid aqueous solution. Water pumped up from the wells for pumping water generally contains a carbonic acid aqueous solution.
The above-mentioned objects are also attained by an agent (cleaning agent) for remedying contaminated soil to be poured into the soil, which comprises a carbonic acid aqueous solution having a carbonate ion concentration of 100 to 10,000 ppm.
The agent for remediation can contain an inorganic salt or a hydrophilic organic solvent.
The above-mentioned carbonate ion concentration means, in the invention, a value obtained by measuring according to a strontium halide/monobasic acid titration method described in JIS K 0101 25.
In the invention, the contaminants are generally water-soluble substances, and the water-soluble substances include those that are capable of dissolving in a slight amount in water, preferably those capable of dissolving in at least 0.01 g (particularly at least 0.01 g) per 1 L(litter) of water in water.